I had suspected sticky steering two years ago, I lubricated the lower UJ's a couple of times a year and had no further problems.
I researched the issue to death at the time and realised you need to consider it in terms of whole steering system and it's interdependent parts.
BMW tech materials refer to the electronic module naturally heating the more it is worked, and that it has a staged thermal cut-out which reduces the power assistance as the temperature climbs to a critical level.
The worm gears are a precision item in sealed for life grease and the column runs through a pair of nylon bushes, again sealed for life.
The lower column UJs are in the engine bay exposed to elements and extreme temperature changes being located near the engine and exhaust.
The EPS is also sensitive to over or under voltage values and will not function correctly in either state.
The EPS control module connectors can be loose, and internal solder joints fail as in any module.
Logically anything that makes the SYSTEM work harder will heat the module and potentially cause it to trip reducing assistance until things cool. Resistance caused by dry or corroded lower UJs or worm gears binding because of dry grease or expansion (due to heat), or dry nylon bearings will require the system to work harder heating the module. External heating of the module due to the cars heater or ambient air will add to this.
A bad battery or other voltage/wiring problem can cause an intermittent EPS malfunction.
Some of the above will register fault codes on the OBD system.
BMW tech updates only refer to the worm gear being re-engineered and no mention is made of a revised module (unless anyone has seen different?). Clearly if you reduce resistance in the system by lubricating the parts mentioned or reduce the external heating of the module symptoms can be eliminated. Which probably explains why lubricating UJs is sometimes enough, the trick of tapping a grease nipple on to the worm gear casing will help and reducing external heating of the module helps for others. It's all part of reducing load and managing temp.
Looking at it methodically I would check for fault codes, ensure connections were good, check battery and alternator values, lube the UJs or replace the lower column and UJs, and try the grease nipple on the worm gear housing. Then and only then would I go for the full column unit. but as I say mines been fine for two years (fingers crossed) :-| .
edit; or use it as an excuse to buy an ///M with it's nice old skool hydraulic steering
